Spirit or bubble levels used to plumb vertical posts, pipes or poles are well known. For example, an early U.S. Pat. No. 392,124 to Jackson describes a spirit level attachable to a vertical post with screws. U.S. Pat. No. 3,826,013 to Baher describes a post level with right-angular arms adapted to engage a circumferential surface of a post. The post level is hand-held against the post and bubble vials indicate the degree of verticality of the post.
U.S. Pat. No. 2,727,314 to Dossie et al. describes a post level which allows plumbing both sides of a vertical corner post simultaneously. This device uses an extended plumbing surface with mirrors to view the bubble vials.
U.S. Pat. No. 5,207,004 to Gruetzmacher describes a post level defined by a pair of right-angle, non-hinged, rigid walls. A horizontal bubble vial is mounted in each wall, and a vertical bubble vial is mounted at the intersection of the walls.
U.S. Pat. No. 4,976,040 to Mish et al. describes a post level with a pair of columnar level bodies attachable to a post with elastic bands.
U.S. Pat. No. 4,343,093 to Eadens describes a post level including a pair of right-angle hinged arms with an orbital bubble vial, known as a "bull's-eye type" bubble vial, mounted on one of the arms. A chain may be wrapped around a post to attach the level thereto.